Ensure calling ToString when auto advance is enable does not advance time (#4269)

* Ensure calling ToString when auto advance is enable does not advance time
Co-authored-by: NNikita Balabaev <xakep139@users.noreply.github.com>

src/Libraries/Microsoft.Extensions.TimeProvider.Testing/FakeTimeProvider.cs

@@ -161,7 +161,7 @@ public override long GetTimestamp()
     /// Returns a string representation this provider's idea of current time.
     /// </summary>
     /// <returns>A string representing the provider's current time.</returns>
-    public override string ToString() => GetUtcNow().ToString("yyyy-MM-ddTHH:mm:ss.fff", CultureInfo.InvariantCulture);
+    public override string ToString() => _now.ToString("yyyy-MM-ddTHH:mm:ss.fff", CultureInfo.InvariantCulture);
 
     /// <inheritdoc />
     public override ITimer CreateTimer(TimerCallback callback, object? state, TimeSpan dueTime, TimeSpan period)

test/Libraries/Microsoft.Extensions.TimeProvider.Testing.Tests/FakeTimeProviderTests.cs

@@ -320,5 +320,29 @@ public void AutoAdvance()
         Assert.Equal(timeProvider.Start + TimeSpan.FromSeconds(1), second);
         Assert.Equal(timeProvider.Start + TimeSpan.FromSeconds(2), third);
     }
+
+    [Fact]
+    public void ToString_AutoAdvance_off()
+    {
+        var timeProvider = new FakeTimeProvider();
+
+        _ = timeProvider.ToString();
+
+        Assert.Equal(timeProvider.Start, timeProvider.GetUtcNow());
+    }
+
+    [Fact]
+    public void ToString_AutoAdvance_on()
+    {
+        var timeProvider = new FakeTimeProvider
+        {
+            AutoAdvanceAmount = TimeSpan.FromSeconds(1)
+        };
+
+        _ = timeProvider.ToString();
+
+        timeProvider.AutoAdvanceAmount = TimeSpan.Zero;
+        Assert.Equal(timeProvider.Start, timeProvider.GetUtcNow());
+    }
 }